Japan has approved the first human-animal embryo that could lead to new sources of organ transplant. Though there are still technical and ethical hurdles. Government of Japan is supporting a stem cell scientist Hiromitsu Nakauchi who leads teams at the University of Tokyo and Stanford University in California for creating animal embryos that contain human cells and transplant them into surrogate creatures as the restriction to practice was overturned this year.
Human cells are planned to be grown in mouse and rat embryos and then it could be transplanted into surrogate animals. Nakauchi said that he wants to produce animals with organs made from human cells which can be eventually transplanted into individuals. He also said that he is planning to grow a hybrid mouse for 14.5 days when the organs are mostly formed, almost to be termed and 15.5 days for the same experiment on rats. He is awaiting government approval for up to 70 days, for growing hybrid embryos in pigs. The study has been published in Nature journal.
Bioethicists are concerned that the human cells might stray beyond the development of the specific organ and travel to the animal’s brain and affect the cognition. He also added that these concerns were taken into consideration during experiments and that he made sure that cells go only to the pancreas and not the entire body.
Scientists are investigating a strategy to make an animal embryo that lacks a gene vital for the generation of a specific organ, like the pancreas and then infuse human induced pluripotent stem (iPS) into the animal fetus. iPS cells have been programmed to an embryonic-like state which can offer ascent to all cell types. As the animal starts to develop, it will make use of the human iPS cells to make an organ which it cannot make on its own.
At the American Association for the Advancement of Science meeting in Austin, Texas in 2018 scientists reported they had put human iPS cells into the sheep embryos and had been engineered not to produce a pancreas but even after growth for 28 days, it contained very less human cells and nothing resembling organs. Nakauchi thinks that is due to the genetic distance between humans and sheep.
Jun Wu who is a researcher at the University of Texas Southwestern Medical Center in Dallas said that it is not useful to bring human-animal hybrid embryos in evolutionarily distant species like pigs and sheep as human cells will be eliminated by the host early on. There is a need to understand the molecular basics and then develop strategies to overcome problems. The approval in Japan will be helpful in experimenting with iPS cells at different stages to find out the limit of growth of human cells in animal embryos.